Lately, two-wire type sensors using only two wires both as power lines and as signal lines are widely used. According to a detection system consisting of such a two-wire sensor and a controller which are connected to each other by two wires, the sensor supplies an on/off signal, and the controller distinguishes the on/off state of the signal according to a certain threshold level.
According to such conventional two-wire type detection systems, since the controller detects the on/off state of the signal according to a certain threshold level, it cannot distinguish an abnormal off state signal which may be due to wire breakage or wire disconnection from a normal off state signal, and, when a shortcircuiting has occurred in the sensor, the controller receives what appears to be an on state signal and, again, cannot distinguish it from a normal on state signal.
One of the most important applications of this technology is found in proximity switches which are typically used in an extremely large number in modern manufacturing plants in conjunction with the use of programmable controllers. In such applications, owing to such advantages as a reduced amount of wiring, the possibility of serial connection to a programmable controller, and the elimination of dedicated power lines for the proximity switches, two-wire type DC proximity switches have been preferred. Such a conventionally known two-wire type proximity switch comprises an oscillation circuit including a detection coil and a resonance capacitor, an integration circuit for integrating the output from the oscillation circuit, a comparator circuit comparing the output from the integration circuit with a reference level, and an output circuit for responding to the output from the comparator circuit, and the two power lines leading to these circuits are also used as signal lines.
Generally, when a proximity switch is used in combination with other sensors and controllers as a part of a large system, various component parts thereof including the proximity switch are required to be highly reliable, and, as a part of such a consideration, each component is desired to be provided with a self diagnosis function. In particular, in proximity switches using detection coils, it is highly desirable to have an output for indicating the occurrence of any breakage in the detection coils. However, according to conventional two-wire type proximity switches, the output consists of only two possible states to indicate the states of on and off, and it has been impossible to obtain a signal output indicating the occurrence of breakage of a coil wire.